Modeling Cell Biological Features of Meiotic Chromosome Pairing

During meiosis, homologous chromosomes become associated side by side in a process known as homologous chromosome pairing. Pairing requires long range chromosome motion through a nucleus that is full of other chromosomes. It remains unclear how the cell manages to align each pair of chromosomes quickly while mitigating and resolving interlocks. Here, we use a coarse-grained molecular dynamics model to investigate how specific features of meiosis, including motor-driven telomere motion, nuclear envelope interactions, and increased nuclear size, affect the rate of pairing and the mitigation/resolution of interlocks. By creating in silico versions of three yeast strains and comparing the results of our model to experimental data, we find that a more distributed placement of pairing sites along the chromosome is necessary to replicate experimental findings. Active motion of the telomeric ends speeds up pairing only if binding sites are spread along the chromosome length. Adding a meiotic bouquet significantly speeds up pairing but does not significantly change the number of interlocks. An increase in nuclear size slows down pairing while greatly reducing the number of interlocks. Interestingly, active forces increase the number of interlocks, which raises the question: How do these interlocks resolve? Our model gives us detailed movies of interlock resolution events which we then analyze to build a step-by-step recipe for interlock resolution. In our model, interlocks must first translocate to the ends, where they are held in a quasi-stable state by a large number of paired sites on one side. To completely resolve an interlock, the telomeres of the involved chromosomes must come in close proximity so that the cooperativity of pairing coupled with random motion causes the telomeres to unwind. Together our results indicate that computational modeling of homolog pairing provides insight into the specific cell biological changes that occur during meiosis.

Meiotic Aberrations Leading To Aneuploidy In Cauliflower (Brassica Oleracea L. var. Botrytis)

Aneuploid cauliflower plants (Brassica oleracea L. var. botrytis) display abnormal curd phenotypes causing serious commercial problems in offspring populations. Despite extensive breeding efforts, selection of genotypes producing euploid gametes remains unsuccessful due to unknown genetic and environmental factors. To reveal the origin of aneuploid gametes, we analyzed chromosome pairing, chiasma formation and chromosome segregation in pollen mother cells of selected cauliflower genotypes. To this end we compared different genotypes exhibiting Low with < 5%, Moderate with 5-10% and High with > 10% aberrant offspring. Microscopic observations revealed regular chromosome pairing at pachytene. However, cells at diakinesis and metaphase I showed variable numbers of univalents, suggesting that chiasma formation during meiotic prophase is incomplete or disrupted and results in a partial desynaptic phenotype. Cells at anaphase I – telophase II exhibited various degrees of unbalanced chromosome numbers explaining the aneuploid offspring. Immunofluorescence probed with an MLH1 antibody demonstrated fluorescent foci in all genotypes, but their lower numbers do not correspond to the putative sites of chiasmata. Interchromosomal connections between chromosomes and bivalents are common at diakinesis and metaphase I, and they contain centromeric and 45S rDNA tandem repeats, but such threads seemed not to affect proper disjoin of the half bivalents at anaphase I. Moreover, male meiosis in the arabidopsis APETALA1/ CAULIFLOWER double mutant with the typical cauliflower phenotype did show interchromosomal connections, but there were no indications for partial desynapsis. We now hypothesize that the occurrence of desynapsis in cauliflower is a developmental out-of-phase phenomenon partially or completely controlled by genes involved in flower and curd development.

Altering the Binding Properties of PRDM9 Partially Restores Fertility across the Species Boundary

Sterility or subfertility of male hybrid offspring is commonly observed. This phenomenon contributes to reproductive barriers between the parental populations, an early step in the process of speciation. One frequent cause of such infertility is a failure of proper chromosome pairing during male meiosis. In subspecies of the house mouse, the likelihood of successful chromosome synapsis is improved by the binding of the histone methyltransferase PRDM9 to both chromosome homologs at matching positions. Using genetic manipulation, we altered PRDM9 binding to occur more often at matched sites, and find that chromosome pairing defects can be rescued, not only in an intersubspecific cross, but also between distinct species. Using different engineered variants, we demonstrate a quantitative link between the degree of matched homolog binding, chromosome synapsis, and rescue of fertility in hybrids between Mus musculus and Mus spretus. The resulting partial restoration of fertility reveals additional mechanisms at play that act to lock-in the reproductive isolation between these two species.

Interfered Chromosome Pairing Promotes Meiosis Instability of Autotetraploid Arabidopsis by High Temperatures

Alterations of environmental temperature affect multiple meiosis processes in flowering plants. Polyploid plants derived from whole genome duplication (WGD) have enhanced genetic plasticity and tolerance to environmental stress, but meanwhile face a challenge for organization and segregation of doubled chromosome sets. In this study, we investigated the impact of increased environmental temperature on male meiosis in autotetraploid Arabidopsis thaliana. Under low to mildly-increased temperatures (5-28°C), irregular chromosome segregation universally takes place in synthesized autotetraploid Columbia-0 (Col-0). Similar meiosis lesions occur in autotetraploid rice (Oryza sativa L.) and allotetraploid canola (Brassica napus cv. Westar), but not in evolutionary-derived hexaploid wheat (Triticum aestivum). As temperature increases to extremely high, chromosome separation and tetrad formation are severely disordered due to univalent formation caused by suppressed crossing-over. We found a strong correlation between tetravalent formation and successful chromosome pairing, both of which are negatively correlated with temperature elevation, suggesting that increased temperature interferes with crossing-over prominently by impacting homolog pairing. Besides, we showed that loading irregularities of axis proteins ASY1 and ASY4 co-localize on the chromosomes of syn1 mutant, and the heat-stressed diploid and autotetraploid Col-0, revealing that heat stress affects lateral region of synaptonemal complex (SC) by impacting stability of axis. Moreover, we showed that chromosome axis and SC in autotetraploid Col-0 are more sensitive to increased temperature than that of diploid Arabidopsis. Taken together, our study provide evidence suggesting that WGD without evolutionary and/or natural adaption negatively affects stability and thermal tolerance of meiotic recombination in Arabidopsis thaliana.

Homologous locus pairing is a transient, diffusion-mediated process in meiotic prophase

The pairing of homologous chromosomes in meiosis I is essential for sexual reproduction and is mediated, in part, by the formation and repair of Spo11-induced DNA double strand breaks (DSBs). In budding yeast, each cell receives ~150-200 DSBs, yet only a fraction go on to form crossover products. How and why the cell initially co-ordinates so many interactions along each chromosome is not well understood. Using a fluorescent reporter-operator system (FROS), we measure the kinetics of interacting homologous loci at various stages of meiosis. We find that while tagged loci undergo considerable motion throughout prophase I, they are constrained in how far they can diffuse from their homolog pair. This effective tethering radius decreases over the course of meiosis in a DSB-dependent manner. We develop a theoretical model that captures the biological contributions of centromere attachment to the nuclear envelope, homolog pairing, and nuclear confinement. With this model, we demonstrate that the experimentally observed heterogeneity in single-cell behavior and the effective tethering between loci is captured for two polymers forming randomly-spaced linkages. The small number of connections required to reproduce our data demonstrates that a single linkage site between homologous chromosomes can constrain the movement of loci up to hundreds of kilobases away.Significance StatementMeiosis is essential for sexual reproduction, and homologous chromosome pairing is a critical step in this process that must be reliably achieved. We measure the dynamics of homologous loci throughout prophase I of meiosis, demonstrating the transient nature of homolog contacts and heterogeneity in single-cell behavior. We develop a minimal model containing only the basic polymer physics of DNA but is sufficient to reproduce the observed behavior. We show that it only takes a handful of homologous linkages per chromosome to facilitate pairing, demonstrating that a single tethered locus can drastically restrict the diffusion of DNA tens to hundreds of kilobases away. These results demonstrate the central role of random diffusion and polymer physics in facilitating chromosome pairing in meiosis.

Patterns and Processes of Diploidization in Land Plants

Most land plants are now known to be ancient polyploids that have rediploidized. Diploidization involves many changes in genome organization that ultimately restore bivalent chromosome pairing, disomic inheritance, and resolve dosage and other issues caused by genome duplication. In this review, we discuss the nature of polyploidy and its impact on chromosome pairing behavior. We also provide an overview of two major and largely independent processes of diploidization: cytological diploidization and genic diploidization/fractionation. Finally, we compare variation in gene fractionation across land plants and highlight the differences in diploidization between plants and animals. Altogether, we demonstrate recent advancements in our understanding of variation in the patterns and processes of diploidization in land plants and provide a road map for future research to unlock the mysteries of diploidization and eukaryotic genome evolution. Expected final online publication date for the Annual Review of Plant Biology, Volume 72 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Homoeologous chromosome pairing at metaphase I of meiosis in Hordeum vulgare L. × H. bulbosum L. triploid hybrids (HvHbHb)

Background. One of the ways to use the genetic potential of bulbous barley, which is characterized by a number of valuable traits, is interspecific hybridization. In crosses of H. vulgare (2x) × H. bulbosum (2x) and H. vulgare (4x) × H. bulbosum (4x) with a genome ratio of 1Hv: 1Hb in a hybrid embryo, elimination of bulbous barley chromosomes is observed in many cases, and the intensity of the process and the result of the crossing depend on the genotypes of the parental forms. This limits the possibility of including a significant variety of parental forms in crosses. Сrossing of diploid forms of H. vulgare with tetraploid accessions of H. bulbosum (4x) results in the formation of triploid hybrids (HvHbHb) with stable chromosomal composition in pollen mother cells (PMCs) at metaphase I (MI) of meiosis. These triploid hybrids can serve as a basis for obtaining series of introgressive lines of cultivated barley. One of the tasks of this type of work is to estimate the involvement of various chromosomes and their arms in homoeologous associations. The aim of this work was to study the possibility of homoeologous pairing of chromosomes of parental species at MI of meiosis in triploid hybrids using GISH and FISH with chromosome-specific markers, as well as to register the participation of individual arms of the cultivated barley chromosomes in homoeologous associations with the chromosomes of bulbous barley in triploid hybrids (HvHbHb).Materials and methods. Seven triploid hybrids of H. vulgare × H.bulbosum (HvHbHb) obtained in four combinations of crosses with the participation of three diploid cultivars of cultivated barley and two tetraploid accession of bulbous barley were used in this study. The features of homoeologous pairing of chromosomes at MI were studied using the method of fluorescent in situ hybridization (GISH and FISH) with chromosome-specific markers.Results All the studied hybrid plants are characterized by a stable chromosomal composition in PMCs at the MI stage of meiosis. Meiotic configurations formed by homoeologous chromosomes of the parental species, ranging from 0.87 to 1.40 on average per cell, were identified in all the studied plants. Among them, vbb trivalents prevailed. Analysis of chromosome pairing at MI in triploid hybrids revealed the participation of all chromosome arms of H. vulgare in homoeologous Hv-Hb associations, except for the short arm of chromosome 1H. In all the studied triploid hybrids, there is a tendency for a higher frequency of involvement of the long arms of chromosomes in the formation of homoeologous associations; this feature is most clearly manifested in case of chromosome 5H.Conclusions Intergenomic associations with the participation of all arms of H. vulgare chromosomes, except for the short arm of chromosome 1H, were revealed at MI in H. vulgare × H. bulbosum triploid hybrids (HvHbHb). Chromosome 5H, as well as any other cultivated barley chromosome, is characterized by a higher involvement of its long arm in homoeologous associations Hv-Hb, as compared to the short arm.